(1) Field of the Invention
The present invention relates to magnetoelectric multilayer composites comprising alternate layers of a magnetostrictive material, which is a bimetal ferrite wherein one of the metals is zinc, and a piezoelectric material such as lead zirconate titanate (PZT), lead zincate niobate (PZN), lead zincate niobate lead-titanate (PZN-PT), lead magnesium niobate lead-titanate (PMN-PT), lead lanthanum zirconate titanate (PLZT), Nb/Ta doped-PLZT, and barium zirconate titanate (BZT) for facilitating the conversion of an electric field into a magnetic field or vice versa. The preferred composites include cobalt, nickel, or lithium zinc ferrite and PZT films which are arranged in a bilayer or in alternating layers, laminated, and sintered at high temperature. The composites are useful in sensors for detection of magnetic fields; sensors for measuring rotation speed, linear speed, or acceleration; read-heads in storage devices by converting bits in magnetic storage devices to electrical signals; magnetoelectric media for storing information; and high frequency devices for electric field control of magnetic devices or magnetic field control of electric devices.
(2) Description of Related Art
Conversion of electric to magnetic fields and vice versa plays an important role in many devices. One way in principle to accomplish this is a composite of magnetostrictive and piezoelectric materials. In such composites, the field conversion is a two step process: magnetostriction induced mechanical deformation resulting in induced electric fields. Until now, interest in such transducers has been lacking because of conversion efficiencies that are an order of magnitude below theoretical predictions.
The magnetoelectric (ME) effect is defined as the dielectric polarization of a material in an applied magnetic field or an induced magnetization in an external electric field (Landau and Lifshitz, Electrodynamics of Continuous Media (Pergamon, Oxford, 1960), p. 119). In materials that are magnetoelectric (ME), the induced polarization P is related to the magnetic field H by the expression, P=αH, where α is the second rank ME-susceptibility tensor. A parameter of importance is the ME voltage coefficient αE=δE/δH with α=ε0εrαE where εr is the relative permittivity. The effect, first observed in antiferromagnetic Cr2O3, is generally weak in single phase compounds (Astrov, Soviet Phys. JETP 13: 729 (1961); Rado and Folen, Phys. Rev. Lett. 7: 310 (1961); Foner and Hanabusa, J. Appl. Phys. 34: 1246 (1963); Tsujino and Kohn, Solid State Commun. 83: 639 (1992); Bichurin, Ferroelectrics 204: 356 (1997); Kornev et al., Phys. Rev. B 62: 12247 (2000)). A strong ME effect, however, could be realized in a “product-property” composite consisting of magnetostrictive (MS) and piezoelectric (PE) phases in which the mechanical deformation due to magnetostriction results in a dielectric polarization due to piezoelectric effects (Suchtelen, Philips Res. Rep. 27: 28 (1972)). Van den Boomgaard synthesized bulk composites of cobalt ferrite or nickel ferrite with BaTiO3 that yielded ME voltage coefficient αE that was a factor 40–60 smaller than calculated values (Van den Boomgaard et al., J. Mater. Sci. 9: 1705 (1974); Van den Boomgaard et al., Ferroelectrics 14: 727 (1976)). Possible causes for such low αE include microcracks due to thermal expansion mismatch between the two phases, leakage current through low resistivity ferrites, porosity, and any impurity or undesired phases.
A multilayer structure is expected to be far superior to bulk composites since the PE layer can easily be poled electrically to enhance the piezoelectricity and the ME effect. In addition, MS layers enclosed in metal electrodes lead to series electrical connectivity for PE layers and further enhancement of piezoelectricity (Harshe et al., Int. J. Appl. Electromagn. Mater. 4: 145 (1993); Avellaneda and Harshe, J. Intell. Mater. Syst. Struct. 5: 501 (1994)). Harshe and co-workers proposed a theoretical model for a magnetostrictive-piezoelectric bilayer structure. The estimated αE for cobalt ferrite (CFO)-lead zirconate titanate (PZT), or -barium zirconate titanate (BZT), bilayer was in the range 0.2–5 V/cm Oe, depending on field orientations, boundary conditions, and material parameters (Harshe, Ph.D. thesis, Pennsylvania State University (1991)). They also prepared multilayers by sintering tape-cast ribbons (Harshe et al., Int. J. Appl. Electromagn. Mater. 4: 145 (1993); Avellaneda and Harshe, J. Intell. Mater. Syst. Struct. 5: 501 (1994)). Samples of CFO—BaTiO3 structures did not show ME effects. The largest αE=75 mV/cm Oe, a factor of 3–30 times smaller than theoretical values, was measured for CFO-PZT. The low αE or the absence of ME effect is most likely due to unfavorable interface conditions and the following problems due to the use of platinum electrodes at the interface: (i) the electrode makes it a three-phase multilayer structure and leads to poor mechanical coupling between the two oxide layers, (ii) platinum with thermal expansion coefficient much higher than that of oxides will result in micro-cracks at the interface during sample processing, (iii) measurement conditions for ME effects might correspond to the inelastic region of stress-strain characteristics for Pt leading to a reduction in ME coefficients.
In summary the use of appropriate MS and PE phases and the elimination of foreign electrodes are critically important for obtaining large ME effects in the multilayer (ML) structures. However, current materials available for making magnetostrictive and piezoelectric composites produce composites which have conversion efficiencies that are an order of magnitude below theoretical predictions. The present invention provides a novel class of materials for making magnetostrictive and piezoelectric composites that have a large ME effect and maximum field conversion efficiency.